The present invention relates to the field of recording head drive mechanisms.
In U.S. Pat. No. 4,342,504 issued to Peter Ebner and assigned to the same assignee as the present invention, a phototypesetter is disclosed having a recording head which is mounted upon a support carriage which in turn is driven by a belt drive mechanism. A flat ribbon of fiber optic elements is coupled at one terminal portion to a matrix of light emitting diodes and the other terminal portion is embedded within a recording head, which presses against the photosensitive paper for the purpose of recording a line of characters upon a single scan of the head across the paper. The fiber optic elements terminating in the recording head form two rows which enable the characters to be formed by recording character segment strips which overlap each other. The recording head is mechanically scanned across the photosensitive paper at a velocity of about 18 inches per second, and in order to accurately generate characters by selectively illuminating the light emitting diodes in the matrix, the exact position of the recording head during scanning must be precisely correlated with the L.E.D. illumination actuation data being generated, in order to accurately reproduce the desired fonts of characters of typeset quality. Thus the recording head drive mechanism ideally should have no backlash or slippage.
Accordingly it is highly desirable to reduce the backlash or slippage of a conventional belt-pulley interface to virtually zero backlash in a relatively inexpensive manner. Early attempts involved the use of urethane pulleys having a high coefficient of friction. However, it was found that the deformation of the pulleys resulted in the above mentioned detrimental backlash. The other approach was to roughen steel pulleys which are substantially non-deformable, to increase the coefficient of friction. Due to paper cutting, paper dust passes through the machine owing to the cooling fan, and such dust after a time tends to reduce the coefficient of friction where a roughened steel pulley is employed.
It is thus desirable to provide a pulley which is substantially non-deformable, in order to eliminate the above mentioned backlash, and yet has a very high, constant coefficient of friction which does not change owing to exposure to dust and dirt. Since the above mentioned character strip elements must be positioned within one half of one thousandth of an inch, regardless of the scanning speed of the head, which is typically in the neighborhood of 18 inches per second, the requirement for extremely small backlash and slippage of the head drive system is of the utmost importance to attain very high quality typesetting with an inexpensive open loop recordation system. The worker in the art in the light of these requirements, would naturally consider the design of relatively expensive closed loop servo systems which maintain fairly accurate control regardless of backlash, slippage and the like. Other approaches such as the use of precision leadscrews are also expensive and too slow for rapid line recordation.